Films about historical atrocities, whether documentaries or not, usually embrace the perspective of victims, survivors or resisters. The dead cry out for commemoration, and the living need to be provided with consoling stories of courage and resilience. Viewers generally prefer to identify with the innocent, the lucky and the brave, rather than with the monstrous but no less human perpetrators of crimes against humanity.

There are, nonetheless, a few movies that try to take us inside the minds and motives of the guilty, and to show us the familiar — the banal — face of evil. Claude Lanzmann’s “Shoah” (recently reissued on DVD by the Criterion Collection) is a notable and still notably rare attempt to explore genocide not only as a historical cataclysm but also as a result of innumerable instances of actual, ordinary behavior.

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In the documentary "The Act of Killing" Anwar, center, with a model of himself, offers lessons in death-squad technique.CreditDrafthouse Films

Though his methods differ from Mr. Lanzmann’s, and his aims are less comprehensive, the 38-year-old filmmaker Joshua Oppenheimer undertakes a similar inquiry in “The Act of Killing,” his dogged, inventive, profoundly upsetting and dismayingly funny documentary about the Indonesian massacres that began in 1965 and claimed, by some estimates, as many as 2.5 million lives over the next year.

If those murders are less well known than others (like the Khmer Rouge slaughter in Cambodia a decade later), it is partly because Indonesia has not undergone the kind of public reckoning that often follows such catastrophes. “History is written by the winners,” muses one of Mr. Oppenheimer’s subjects, who led a right-wing death squad in North Sumatra. “And we are the winners.”

He and his former colleagues, who still keep in touch and occasionally socialize with one another, lead comfortable, even privileged lives. Some serve in provincial or national governments. They speak at rallies of Pancasila Youth, a nationalist paramilitary group still apparently popular — and also still feared — for its role in wiping out suspected Communists almost 50 years ago.

Perhaps the most startling thing about the killers in “The Act of Killing” is that they seem to have no interest in denying, excusing or minimizing their crimes. On the contrary, they are candid, even boastful about what they have done, and eager to share their recollections of torture and murder. “Never forget” is traditionally the slogan of victims fending off revisionism, indifference and the passage of time, but in this case the killers themselves seem most interested in keeping the memory alive.

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Herman, another killer, likes to dress up as movie archetypes.CreditDrafthouse Films

The way they do this is, to say the least, unusual, both in the annals of barbarity and the history of cinema. Mr. Oppenheimer focuses on two men, Anwar and Herman, self-described gangsters who enthusiastically devote themselves to the project of making a movie about the brutal campaigns against Communists, ethnic Chinese and critics of the military government that overthrew President Sukarno in 1965. But instead of putting together the usual archival or talking-head documentary, they and Mr. Oppenheimer collaborated on a series of elaborate re-enactments, using makeup, costumes and special effects to evoke the terror and (for Anwar and Herman, the glory) of the past. They become the stars and auteurs of a grandiose spectacle, in relation to which “The Act of Killing” stands as a deadpan making-of documentary.

Sometimes Anwar, a slender, dapper grandfather with a jaunty walk and a sly, disarming smile, offers straightforward lessons in death-squad technique. Beating a suspected Communist to death produced too much blood, so Anwar preferred strangling his victims with wire. At other times he and Herman dress up like movie gangsters or other movie archetypes. Herman, a stout, excitable man who runs for Parliament on a more or less overt platform of bribery and extortion, likes to wear dresses that evoke Mae West by way of Divine.

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A scene from the documentary “The Act of Killing,” about an Indonesian death squad leader re-enacting his crimes.Published OnJuly 17, 2013

The garish absurdity of some of their performances — there is a musical number in front of a waterfall and another in which dancers parade from the mouth of a giant fish — does not distract from the horror. On the contrary, the way that Anwar and Herman use the fantasy language of genre movies as a tool of stylized, half-earnest confession is what grounds this film’s moral inquiry.

Mr. Oppenheimer’s camera observes the rehearsal and shooting of scenes that matter-of-factly evoke unspeakable moments of cruelty and that occasionally arouse powerful emotional responses in participants. At one point, a younger man who has been enlisted to play a torture victim relates the gruesome story of his stepfather’s abduction and murder by, one suspects, his fellow cast members. He tells the tale as if it were a joke, chuckling and assuring the others that he isn’t blaming or criticizing them, and he goes on to give a performance that is almost unbearably credible. Other re-enactments are so intense and realistic that they terrify the participants and cause some of the Pancasila Youth old-timers to marvel at their own sadism.

The word gangster, as Anwar understands it, means “free man.” And through most of the film, he seems free of inhibitions or remorse. When he and Herman appear on a television news program, the host chronicles their homicidal exploits with a chipper smile, and the audience applauds when she tallies the numbers of the dead.

The horror of “The Act of Killing” does not dissipate easily or yield to anything like clarity. Some queasiness may linger at the thought of a Western filmmaker indulging the creative whims of mass murderers, exploiting both their guilelessness and the suffering of Indonesians who remain voiceless and invisible here. But this discomfort is an important indicator of just how complicated, how perverse, the cinematic pursuit of truth can be. This is not a movie that lets go of you easily.